When we were first learning about it, there were some misconceptions about radioactivity and health. There were even business minded individuals who widely sold it as a miracle cure. This public belief was reinforced by the fact that around that time we discovered hot springs have radioactive elements, (and people have always believed hot springs heal your ailments) which lead to a mass conception radioactivity was actually a miracle cure. A large part of that down fall was when the "Radium Girls" started literally dying because they were told it was totally safe to work with radioactive material, began falling apart and then worked for legal pushback.

I'm not an expert on the matter, so I might be a little off but that is a good overview on why some people have that belief still. As always it's shitty people looking to make money off of hype. The Radium Girls had a tragic but ultimately fascinating life/story. They would even rub the material on their teeth to glow. Check it out if you're interested.